Clustering is a process of grouping a set of objects into a plurality of classes each composed of similar objects, that is, a process of classifying objects into different classes (clusters), such that objects in the same class have a relatively large similarity, and objects in different classes have a relatively large dissimilarity. In this context, “class” and “cluster” have the same meaning, as used herein.
A conventional clustering method can be used to classify human face images, so as to classify images of a same person into one class. A Rank-Order distance can be used in the conventional clustering method to measure a similarity between two human faces in images, and images of the same person can be clustered together. However, if a number of faces contained in a group of images is relatively large, and the number of images of each person is relatively small, accuracy of clustering results by the conventional clustering method can be low.